Apparently, old-time tradition dictates that two heirs to the throne should never travel on the same plane together, in case something happens and the country ends up losing several members of the line of succession. So strictly speaking, William, who is second-in-line to inherit the crown behind his father Charles, should never fly with with George, who is third, or Charlotte, who is fourth.

The Duke and Duchess of Cambridge arriving in Poland with Prince George and Princess Charlotte in July 2017

However, William and Kate have been ‘breaking’ this royal rule since Prince George joined them on his first official visit to Australia and New Zealand in 2014, when he was just eight months old.

At the time, it was said that the couple had asked The Queen for permission to take their little one on the flight with them, which was duly granted.

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‘They had to ask the Queen for permission but she said yes,’ the Royal press office told BBC’s Newsround at the time.

‘While there is no official rule on this, and royal heirs have travelled together in the past, it is something that the Queen has the final say on.’

Fifth in line behind William’s children is their uncle Prince Harry, followed by the Duke of York Prince Andrew, and then his daughters, Princess Beatrice and Princess Eugenie.

When Princess Charlotte was born in May 2015, she became the first royal daughter with the right to take precedence over any future younger brothers, after laws giving sons priority were changed in 2011.